Emission spectroscopic processing apparatus and plasma processing method using it

ABSTRACT

A plasma processing method using a spectroscopic processing unit which includes separating spectrally plasma radiation emitted from a vacuum process chamber into component spectra, converting the component spectra into a time series of analogue electric signals composed of different wavelength components at a predetermined period, adding together analogue signals of the different wavelength components, converting a plurality of added signals into digital quantities on a predetermined-period basis, digitally adding together the plurality of added and converted signals a plural number of times on a plural-signal basis, determining discriminatively an end point of a predetermined plasma process on the basis of a signal resulting from the digital addition step, and terminating the predetermined plasma process.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 10/659,394, filedSep. 11, 2003, now U.S. Pat. No. 6,890,771, which is a divisionalapplication of U.S. application Ser. No. 10/090,759, filed Mar. 6, 2002,now U.S. Pat. No. 6,716,300, the subject matter of which is incorporatedby reference herein, and the present invention is related to (1) U.S.patent application Ser. No. 09/793,624 filed Feb. 27, 2001, (2) U.S.patent application Ser. No. 09/797,601 filed Mar. 5, 2001 and (3) U.S.patent application Ser. No. 09/946,504 filed Sep. 6, 2001.

BACKGROUND OF THE INVENTION

The present invention relates generally to an emission spectroscopicprocessing apparatus for spectrally separating radiation emitted from aplasma or the like into component spectra, converting the componentspectra having respective wavelengths into electric signals by means ofassociated light receiving elements, respectively, and obtaining adesired detection output by processing the signals. Further, the presentinvention relates to a plasma processing method using the emissionspectroscopic processing apparatus.

The emission spectroscopic processing apparatus for spectrallyseparating radiation emitted from a plasma or the like into componentspectra, converting the component spectra having respective wavelengthsinto electric signals by means of light receiving elements or devicesand obtaining a desired detection output by processing the signals hasbeen known heretofore. By way of example, a process monitoring apparatusadopting a main component analysis process mentioned below is disclosed,for example, in European Patent No. 1089146.

More specifically, electromagnetic radiation emitted from a plasmacamber is inputted into a process monitoring apparatus which is composedof a spectrometer and a processor through the medium of an optical fiberor the like. The spectrometer mentioned above is designed to spatiallysplit or separate the electromagnetic radiation of plasma on the basisof the wavelengths by using, for example, a prism or a diffractiongrating. Subsequently, a plurality of spatially separated spectra ofrespective wavelengths are detected by means of e.g. a CCD (ChargeCoupled Device) array of 2048 channels, whereby a detection signal,i.e., OES (Optical Emission Spectroscopic) signal is generated. The OESsignal is then digitized through e.g. an analogue-to-digital (A/Dconverter) to be outputted to a processor for undergoing furtherprocessings. In this manner, the electromagnetic radiation emitted froma plasma is measured by the spectrometer and supplied to the processorin the form of the OES signal of 2048 channels.

Main or major component analysis process of a specific or desired typeto be executed by the processor is selected by means of a remotecomputer system, fabrication equipment or the like. In place of thespectrometer, there may be employed a diffraction grating, prism,optical filter or other type of wavelength selecting device(s) incombination with a plurality of detectors (e.g. photodiodes, photomultipliers or the like) to thereby supply the information concerning aplurality of electromagnetic radiation wavelengths to the processor. Inthis conjunction, it is to be added that the processor is coupled to aplasma etching controller by way of a control bus.

SUMMARY OF THE INVENTION

In practice, the CCD array is used in many applications as theconvenient means for making available radiation amplitude signalscorresponding to the wavelengths of the component spectra. However, inthe CCD array which is constituted by a large number of integrated lightreceiving elements, such a problem is encountered that when the lightreceiving elements each of small capacity are employed in an effort toincrease the sensitivity, then noise also increases, whereas when thelight receiving elements of large capacity are employed with a view tosuppressing the noise, the sensitivity of the CCD array will becomelowered. By way of example, in the case of a CCD array of a relativelyhigh sensitivity (e.g. 2048-pixel CCD linear sensor “ILX511”commercially availably from Sony Co. Ltd), the signal-to-noise ratio(S/N ratio) is on the order of 250 in the state where a quantity oflight of saturation level is received, and the S/N ratio decreases inproportion to the one-second power (1/2) of the received light quantityas it deceases. This problem is not inherent to the CCD array butgenerally common to photosensor devices each composed of a large numberof integrated light receiving elements.

In the ordinary image sensor, a mean value of the received lightquantity distribution over the whole image or a peak value thereof ismeasured for the purpose of effectuating a gain adjustment for changingthe amplification factor for the output signal of the CCD array or thecharge storing time thereof on the basis of the measured value in orderto cope with changes or variations in the quantity of incident light orradiation. In this conjunction, reference may be made to, for example,Japanese Patent Application Laid-Open Publication No. 324297/2000 andUSP 2001/0016053A1.

On the other hand, in the plasma processing apparatus, the incidentradiation quantity may change remarkably (ca. ten times or more) due toaged contamination of a process chamber. For coping with such change ofthe incident radiation quantity, it is not preferred to change thecharge storing time of the CCD array because then the operation timingof the whole system will have to be changed remarkably. Further, in thespectrum of plasma emission produced in the plasma processing apparatus,there are coexistent mixedly a plurality of high luminance portionsexhibiting steep peaks and low luminance portions changing relativelygently as a function of the wavelength (see e.g. U.S. Pat. No.6,261,470B1, FIG. 17A or European Patent No. 1089146, FIG. 3C). In theapplications where the emission spectra are detected by using the CCDarray, setting of the amplification factor for the output signal of theCCD array so that no saturation can occur at the steep peaks willinevitably be accompanied with remarkable degradation of the S/N ratiofor the low luminance portion of the radiation. On the contrary, whenthe amplification factor is set in conformance with the low luminanceportion, saturation will easily occur in the peak portions.

In a semiconductor fabrication apparatus, the time-dependent changes ofemission spectra emitted from a process chamber of the apparatus (i.e.,change of the emission spectra in the course of times lapse) indicatechanges in the contents of processing or treatment being carried outwithin the chamber. In recent years, it has been practiced to estimatethe process situation or statuses within the process chamber on thebasis of extremely small or minute changes of the emission spectra.However, in the case where the CCD array or the like device is used asthe means for detecting the emission spectra, the signal which can bemade available is very poor in respect to the S/N ratio as mentionedpreviously. Such being the circumstances, addition of the signal of asame wavelength is repeated a number of times in an effort to eliminatethe noise components. However, with this method, it is necessary torepetitively perform the addition of a sample one hundred times or moreif the signal-to-noise ratio is to be increased by one order ofmagnitude. Such processing will ordinarily require several to severalten seconds, which in turn renders it relatively difficult to detect theminute change (change of less than 10% or so) of high rate or speed onthe order of one second or 0.5 second or lower in terms of temporalduration. In particular, in a low luminance portion which changesrelatively gently as a function of the wavelength in the emissionspectra such as of plasma, great difficulty will be encountered indetecting the minute change of high rate on the order of one second orless with satisfactory reproducibility.

In the light of the state of the art described above, it is an object ofthe present invention to provide an emission spectroscopic processingapparatus which is capable of detecting minute changes in emissionspectra of high rate or speed on the order of one second or less withenhanced or improved reproducibility.

Another object of the present invention is to provide a plasmaprocessing method which is carried out by using the emissionspectroscopic processing apparatus mentioned above.

In view of the above and other objects which will become apparent as thedescription proceeds, there is provided according to an aspect of thepresent invention an emission spectroscopic processing apparatus whichincludes a spectroscope for spectrally separating input light emittedfrom a process unit into component spectra, a light receiving unitincluding a series of light receiving elements for detecting lightquantities of the component spectra on a wavelength-by-wavelength basis,a first signal hold circuit for holding sequentially each of detectionsignals outputted from subsets of adjacent light receiving elementscontained in said series of light receiving elements for a first period,respectively, an adder unit for adding together the detection signals ofadjacent light receiving elements of the light receiving unit inclusiveof the held detection signals of the subset of the adjacent lightreceiving elements, a second signal hold unit for holding sequentiallysum outputs of the adder unit, and a signal processing unit fordetermining a state of the process unit on the basis of the output ofthe second signal hold unit.

In a preferred mode for carrying out the invention, the emissionspectroscopic processing apparatus includes a light receiving unitcomprised of a series of light receiving elements for detecting lightquantities of the component spectra on a wavelength-by-wavelength basis,an adder unit for adding together the detection signals outputted fromlight receiving elements which correspond to a set of emissionwavelengths intrinsic to preset light emission materials, respectively,a third signal hold unit for holding sequentially sum outputs of theadder unit, and a signal process unit for determining a state of theprocessing unit on the basis of the output of the third signal holdunit.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of the description which follows, reference is made to thedrawings, in which:

FIG. 1 is a block diagram showing schematically and generally astructure of an emission spectroscopic processing apparatus according toan embodiment of the present invention;

FIG. 2 is a block diagram showing schematically and generally astructure of an emission spectroscopic processing apparatus according toanother embodiment of the present invention;

FIG. 3 is a block diagram showing schematically and generally astructure of an emission spectroscopic processing apparatus according toyet another embodiment of the present invention;

FIG. 4 is a block diagram showing schematically and generally astructure of an emission spectroscopic processing apparatus according tostill another embodiment of the present invention;

FIG. 5 is a block diagram showing schematically and generally astructure of an emission spectroscopic processing apparatus according toa further embodiment of the present invention;

FIG. 6 is a block diagram showing schematically and generally astructure of a signal processing unit incorporated in an emissionspectroscopic processing apparatus according to an embodiment of thepresent invention;

FIG. 7 is a flow chart for illustrating a digital signal processingprocedure according to an embodiment of the present invention;

FIG. 8 is a view showing, by way of example, relations betweenintensities and wavelengths of emission spectra together with a maskfunction in an emission spectroscopic processing apparatus according toan embodiment of the present invention;

FIG. 9 is a view for graphically illustrating, by way of example,time-dependent changes of intensities of emission spectra;

FIG. 10 is a flow chart for illustrating a digital signal processingprocedure according to a further embodiment of the present invention;and

FIG. 11 is a flow chart for illustrating a digital signal processingprocedure according to a still further embodiment of the presentinvention.

DESCRIPTION OF THE EMBODIMENTS

The present invention will be described in detail in conjunction withwhat is presently considered as preferred or typical embodiments thereofby reference to the drawings. In the following description, likereference characters designate like or corresponding parts throughoutthe several views.

FIG. 1 is a block diagram showing schematically and generally astructure of the emission spectroscopic processing apparatus accordingto an embodiment of the present invention. Referring to the figure,plasma emission produced within a treatment or process chamber of aplasma process unit 1 is introduced as input light into a spectroscope 3through the medium of an optical fiber 2 via a slit. The spectroscope 3serves for spectrally separating the input light passed through the slitinto component spectra covering angles mutually differing on awavelength-by-wavelength basis. The component spectra resulting from thespectral separation mentioned above then impinge onto a CCD (ChargeCoupled Device) array 4 which incorporates an array or series of plurallight receiving elements (ordinarily in a number ranging from severalhundreds to several thousands, being however presumed in the followingdescription that the number of the light receiving elements is 2048,only by way of example). Thus, a specific light receiving element(detecting element) disposed at a predetermined position in the CCDarray or device 4 is capable of detecting the spectral intensity of aspecific wavelength component of the incident radiation or light.

A timing generating circuit 5 is designed for generating a CCD resettiming signal and a CCD transfer clock signal. The CCD reset timingsignal is effective for determining a storage time of electric chargesstored in the CCD array, while the CCD transfer clock signal determinesa transfer rate of time-serial signals outputted in time series from theCCD array 4. In the description which follows, both the signalsmentioned above will collectively be referred to as the CCD drive signal6, only for the convenience of description. Thus, the CCD array 4 isdriven by the CCD drive signal 6, as a result of which plasma emissionspectral wavelength distribution is outputted as the time-serial signalfrom the CCD array 4 periodically at a predetermined time interval. Insuccession, the time-serial signal is inputted to an amplifier circuit 7which is imparted with an offset adjusting function and a gain adjustingfunction. At this juncture, it should be mentioned that in the case ofthe conventional emission spectroscopic processing apparatus knownheretofore, the output of the amplifier circuit 7 is directly inputtedto a signal processing unit 9 constituted by a CPU (Central ProcessingUnit) and others via an analogue-to-digital converter (hereinafterreferred to as the A/D converter), whereby the wavelength distributionof the input light, time-dependent changes of light intensity atpredetermined wavelengths and others are displayed on displaying devicesincorporated in the signal processing unit 9.

By contrast, in the case of the emission spectroscopic processingapparatus according to the instant embodiment of the present inventionnow under consideration, the time-serial output signals delivered fromthe amplifier circuit 7 (as obtained by operating sequentially andrepetitively the adjacent CCDs are stored in a plurality n of firstsignal hold circuits 10 (where n≧2) at different timings. The outputsfrom the plurality of first signal hold circuits 10 and that of theamplifier circuit 7 are added together by means of an adder amplifiercircuit 11 the output of which is transferred to a second signal holdcircuit 12 at a predetermined timing. In this manner, a plurality ofsignals corresponding to a plurality of different adjacent timings(i.e., a plurality of different adjacent wavelengths) are addedtogether, to be outputted from the second signal hold circuit 12. Theoutput signal from the second signal hold circuit 12 is then convertedinto a digital signal by means of the A/D converter 8 to be subsequentlyinputted to the signal processing unit 9.

In this way, by adding together the (n+1) signals (where n representsthe number of the first signal hold circuits) by means of the firstadder amplifier circuit 11, the S/N ratio of the output signal of theCCD array 4 can be improved by a factor of √{square root over (n+1)}while the amount of data inputted to the A/D converter 8 can bedecreased by 1/(n+1). By performing this addition processingsequentially for the adjacent CCDs (i.e., for each of (n+1) adjacentCCDs), influence of noise inputted to the signal processing unit 9 cansignificantly be reduced or suppressed.

More specifically, in the case where the number n of the first signalhold circuits 10 is “8”, the signal-to-noise ratio (i.e., S/N ratio) isimproved by a factor of ca. “3” (i.e., three times). Similarly, when thenumber n of the first signal hold circuits 10 is “16”, the S/N ratio isthen improved by a factor of ca. “4”. Further, when the number n of thefirst signal hold circuits 10 is “32”, the S/N ratio can be improvedapproximately by a factor of “6”. In this conjunction, it should bementioned that an integrated circuit of a standard size in which eightsignal processing circuits are integrated has already been commerciallyavailable. Accordingly, the size of the circuit portion comprised of thefirst signal hold circuits provides no serious problems in practicalapplications.

At this juncture, it is to be noted that since the data quantity isdecreased through the analogue addition processing performed by theadder amplifier circuit 11 mentioned above, some difficulty will beencountered in analyzing the incident radiation with a high resolution(high wavelength resolution) without taking some appropriate measures.

For coping with the problem mentioned above, in the emissionspectroscopic processing apparatus now under consideration, sucharrangement can be adopted that when high resolution (wavelengthresolution) is required,) the output of the amplifier circuit 7 and thatof the second signal hold circuit 12 mentioned above are inputted to ananalogue switch circuit 13. Thus, when the analysis of high resolution(wavelength resolution) is required, the analogue switch circuit 13 ischanged over to the position for receiving the output of the amplifiercircuit 7 in response to a command issued from the signal processingunit 9 so that the output of the amplifier circuit 7 can directly beinputted to the signal processing unit 9 by way of the AD(Analogue-to-Digital) converter 8. By virtue of this arrangement, it ispossible to changeover with a single means (the analogue switch circuit13) the mode in which the S/N ratio (resolution) is high with thewavelength resolution being relatively low on one hand and the mode inwhich the wavelength resolution is high with the S/N ratio beingrelatively low on the other hand.

FIG. 2 is a block diagram showing schematically and generally astructure of the emission spectroscopic processing apparatus accordingto another embodiment of the present invention. The emissionspectroscopic processing apparatus now under consideration primarilydiffers from that shown in FIG. 1 in the respect that the circuitportion corresponding to the first signal hold circuit generally denotedby 10 in the apparatus shown in FIG. 1 is replaced by a cascadeconnection in which a first stage signal hold circuit generally denotedby 10A including n (n≧2) signal hold circuits (10-11 to 10-1n) and asecond stage signal hold circuit generally denoted by 10B including m(m≧2) signal hold circuits (10-21 to 10-2m) are connected in cascadethrough the medium of an interposed adder amplifier circuit 11-1. Withthis circuit arrangement, it is possible to improve the S/N ratio by afactor of [(n+1)*(m+1)]^(1/2) with a relatively small number (n+m) ofthe signal hold circuits (10-11, . . . , 10-2n). By way of example, inthe case where n=8 with m=8, the S/N ratio can be improved approximatelyby a factor of “9” (i.e., the S/N ratio can be improved about as high asnine times).

FIG. 3 is a block diagram showing schematically and generally astructure of the emission spectroscopic processing apparatus accordingto yet another embodiment of the present invention. The emissionspectroscopic processing apparatus now concerned is designed to analyzesimultaneously the emission spectra from two process chambersincorporated in the plasma process unit 1. To this end, the opticalfiber, the light quantity adjusting device, the spectroscope, the CCDarray, the amplifier circuit, the first signal hold circuit, the adderamplifier circuit and the second signal hold circuit are provided eachin a pair, respectively. Namely, they are the optical fibers 2-1 and2-2, the light quantity adjusting devices 14-1 and 14-2, thespectroscopes 3-1 and 3-2, the CCD arrays 4-1 and 4-2, the amplifiercircuits 7-1 and 7-2, the first signal hold circuits 10-1 and 10-2, theadder amplifier circuits 11-1 and 11-2 and the second signal holdcircuits 12-1 and 12-2. On the other hand, since the same CCD drivesignal 6 is applied in common to the pair of CCD arrays 4-1 and 4-2, thecircuit configuration can correspondingly be simplified. Thus, it issufficient to provide one timing generating circuit 5 and one A/Dconverter 8.

In the emission spectroscopic processing apparatus according to theinstant embodiment of the invention, it is required to digitize twosignals with only one A/D converter 8. Accordingly, the outputs of thesecond signal hold circuits 12-1 and 12-2 are time-serially multiplexedby the time division multiplexing circuit 21, whereon the output of thetime division multiplexing circuit 21 is supplied to the A/D converter 8by way of the analogue switch circuit 13. In this conjunction, it shouldhowever be added that the time division multiplexing circuit 21 may bespared by inputting directly the outputs of the second signal holdcircuits 12-1 and 12-2 to the analogue switch circuit 13 and performingthe A/D conversion while selecting alternately the outputs of the secondsignal hold circuits 12-1 and 12-2 under the command of the signalprocessing unit 9. By way of example, in the case where n=16, the amountof the signal deceases by a factor of 1/9. Consequently, with thealternate A/D conversion mentioned above, the A/D conversion speed orrate can be reduced to ⅛ when compared with the single radiation beaminput system adopted in the conventional emission spectroscopicprocessing apparatus.

Further, with the structure of the emission spectroscopic processingapparatus according to the instant embodiment of the invention, it ispossible to analyze simultaneously the spectral emissions from fourprocess chambers incorporated in the plasma process unit 1. Also in thiscase, the A/D conversion speed or rate can be lowered when compared withthe single radiation beam input system. Thus, the A/D converter as wellas the signal processing unit which are relatively inexpensive can beemployed. This presents significant advantage when a plurality ofradiation input processings are concurrently performed by resorting tothe use of the adder amplifier circuit 11 or adder amplifier circuits11-1 and 11-2. Moreover, in the case where a measured light and areferenced light both derived from the output of the process unit areemployed to be measured by two different CCD arrays, respectively, it ispossible to make zero the difference in the sampling time for the dataat corresponding wavelength between the CCD arrays mentioned above bydriving these CCD arrays in a same timing, whereby arithmetic operationsin which the radiation for measurement and the reference light beam ofrespective wavelengths are used can be performed with high accuracy. Inparticular, when light which changes frequently such as typified by theplasma radiation is used for measurement reference, respectively, thearrangement for driving the plurality of CCD arrays in a same timing asdescribed above will involve great advantages.

On the other hand, in the case where a plurality of CCD arrays areoperated in a same timing as described above, the charge storage timesof the individual CCD arrays become naturally same among them, making itdifficult to adjust the sensitivity of the CCD arrays individually andindependently. Under the circumstances, when the levels of radiationsimpinging onto a plurality of CCD arrays differ remarkably from one toanother in particular, it is preferred to dispose light quantityadjusting devices 14 between the plasma process unit 1 and the opticalfibers 2 or alternatively in the optical fibers 2 or alternativelybetween the optical fibers 2 and the spectroscope 3 while the commandissued from the signal processing unit 9 is converted into an analoguequantity by means of a light quantity setting D/A converter 22 with theanalogue quantity thus derived being then used for controlling the lightquantity adjusting device 14 by way of a light quantity control unit 23,as shown in FIG. 3. Incidentally, as the light quantity adjusting device14 mentioned above, there may be employed a liquid crystal element whosetransmission light quantity varies in dependence on the applied voltageor alternatively a diaphragm mechanism whose optical aperture changes independence on the applied voltage or the like.

In the foregoing, description had been made concerning improvement ofthe S/N ratio and reduction of the data quantity owing to the analogueaddition effectuated by using the adder amplifier circuits 11 andothers.

In this conjunction, it is noted that the S/N ratio can further beimproved by adopting additionally or in combination the digitalprocessing executed by the signal processing unit 9, which will bedescribed below by taking as an example the spectroscopic processingunit shown in FIG. 3 designed for processing two radiation inputs.

It is assumed that the storage time is 25 milliseconds. On thisassumption, it is further presumed that for 128 signals of the samechannel (same wavelength) inputted every 25 milliseconds, 16 adjacentsignals are added together by the adder amplifier circuits 11 to therebyobtain 128 analogue signals for each wavelength (i.e., on awavelength-by-wavelength basis). The analogue signal is then convertedinto the digital signal by the A/D converter 8, the output of which isthen inputted to the signal processing unit 9. In the signal processingunit 9, the signal for each wavelength inputted every 25 milliseconds isadded sixteen times between the adjacent wavelengths and at everysampling. In this way, the signal undergone the average processing oversixteen adjacent wavelengths as well as the signal undergone the averageprocessing through sixteen samplings can be obtained for each ofwavelengths. On the basis of these signals, the desired signalprocessing is carried out, whereby the desired signals which correspondto the two process chambers, respectively, and whose S/N ratios aresignificantly improved can be obtained every 0.5 millisecond.Furthermore, on the basis of these signals, the end points of theprocessing in the two process chambers of the plasma process unit 1 canbe found individually and independently from each other.

In this case, the S/N ratio can be improved by a factor of about “4”through the analogue addition. Additionally, the S/N ratio can furtherbe improved by a factor of about “4” through the wavelength additionprocessing executed by the signal processing unit 9. Moreover, the S/Nratio can be improved by a factor of about “4” through additionprocessing at every sampling point in the signal processing unit. Afterall, the S/N ratio can be improved by a factor of about “64=4*4*4”.

In the case where the S/N ratio of the CCD array 4 is 250 in full scale,the S/N ratio of the radiation signal of 1/64 of the full scaleundergoes degradation by about “30=250/√{square root over (64)}”.However, by effectuating the addition average processing describedabove, the S/N ratio can be restored to about “1900=30*64”. Thus, even aminute change (e.g. 1%) of a low intensity radiation signalcorresponding to 1/64 of the full scale can be separated into about 20levels or gradations.

Although the foregoing description has been made without taking intoaccount the quantization noise involved in the A/D conversion, it isnoted that noise can no more be neglected in the case of the minutesignal on the order of 1/64 of full scale, because in this case the S/Nratio of the signal will be degraded due to noise or the likedisturbance brought about in the A/D conversion. By way of example,considering the case where the conversion to the digital signal isperformed by using the A/D converter of 12 bits, noise inclusive ofquantization noise and noise in other circuits will amount to ca. 1/3000to 1/2000 of the full scale. By taking this into consideration, it canbe said that the S/N ratio of the light or radiation signal of 1/64 ofthe full scale will be lowered less than a half of the value mentionedabove.

FIG. 4 is a block diagram showing schematically and generally astructure of the emission spectroscopic processing apparatus accordingto still another embodiment of the present invention. The spectroscopicprocessing unit now under consideration is so designed as to change theamplification degree or factor on a wavelength basis upon analogueaddition processing. Owing to this arrangement, influences of thequantization noise and the circuit system noise can be reduced.

Upon inputting of the gain setting command for the adder amplifiercircuit 11 to the timing generating circuit 5 from the signal processingunit 9, the timing generating circuit 5 responds thereto by startingafter the CCD reset timing signal the operation of the gain setting A/Dconverter 15 (which should preferably be imparted with a sample-and-holdfunction) in the timing at which the output signal of the adderamplifier circuit 11 (whose gain is set to one (unitary)) is stored inthe second signal hold circuit 12. In this conjunction, it should bementioned that an inexpensive small-size A/D converter capable ofoutputting a digital signal of less than 8 bits inclusive (e.g. about 4or 5 bits) may be used as the gain setting A/D converter 15 without anyproblem. In response to the signal supplied from the timing generatingcircuit 5, an address corresponding to the wavelength is set in anaddress circuit 16, and the digital signal outputted from the gainsetting A/D converter 15 and indicative of the magnitude of the signalis stored in a memory 17 at a corresponding address thereof.

When the operation described above is performed for one storage timeperiod of the CCD, information concerning the magnitudes of the signalswhich correspond to 2048/(n+1) wavelengths are stored in the memory 17.In succession, upon inputting of the gain-affixed data output commandfrom the signal processing unit 9 to the timing generating circuit 5,the amplification factor of the adder amplifier circuit 11 is set incorrespondence to the information concerning the magnitude of the signalin the memory 17 after the CCD reset timing signal. At that time point,the gain of the adder amplifier circuit 11 is set by way of a gainsetting circuit 18 for each of the 2048/(n+1) wavelengths.

The relations between the signal size information mentioned above andthe gains of the adder amplifier circuit 11 should preferably be set asfollows.

Signal Size Information Gain of Adder (relative to full scale) AmplifierCircuit 11 1) from ¼ to 1 (by) A times 2) from ⅛ to ¼ exclusive 2A times3) from 1/16 to ⅛ exclusive 4A times 4) from 1/32 to 1/16 exclusive 8Atimes 5) from 1/32 exclusive 16A times

Incidentally, the value of “A” is ordinarily set to be smaller than “1”(e.g. 1/(n+1), where n represents the number of the first signal holdcircuits).

Ordinarily, the spectral signal originating in the plasma emission doesnot undergo remarkable changes during a single process of specimentreatment. Accordingly, there will usually arise no problem by settingonce the gain of the above-mentioned adder amplifier circuit 11 in thestable discharging state at an earlier stage of the process or specimentreatment. However, the signal mentioned above includes a region inwhich more significant quantization bits are subjected to change due tominute variation of the analogue signal. Such being the circumstances,it is preferred to set the gain of the adder amplifier circuit 11 with amargin so that the adder amplifier circuit 11 is not saturated even whenthe signal of a wavelength concerned increases (generally by ca. 1.3times or more) in the course of a single process or specimen treatment.

Parenthetically, the gain setting data for the adder amplifier circuit11 undergoes conversion to the analogue signal by means of a gain outputcircuit 19 and a third signal holding circuit 20 to be outputted to theanalogue switch circuit 13 time-serially in the same timing as theoutput of the second signal hold circuit 12. Thus, the gain setting datamentioned above can be read by way of the A/D converter 8 under thecommand of the signal processing unit 9. As mentioned previously, thegain setting for the adder amplifier circuit 11 may be performed onlyonce in a stable discharging state at an earlier stage of the process orspecimen treatment. Accordingly, reading of the gain data mentionedabove may be performed only once in the stable discharge state at anearlier stage of the process.

In this manner, the signal processing unit 9 is capable ofarithmetically determining in continuation the true value for each ofthe wavelengths by performing the arithmetic operation on the samewavelengths during the plasma process by using the output data of thethird signal holding circuit 20 of the single time-serial gain settingdata as set and the output data of the second signal hold circuit 12outputted time-serially on a storage-time basis during a single process.

At this juncture, it should be mentioned that in the case where only theminute temporal change of the emission spectra during the single processis subjected to the detection, the above-mentioned arithmetic operationcarried out by means of the signal processing unit 9 by using the gainsetting data is not necessarily required. Further, in conjunction withthe instant embodiment of the invention, it has been described that thegain for the adder amplifier circuit 11 is set only once in the stabledischarging state at an earlier stage of the process. However, it goeswithout saying that the gain for the adder amplifier circuit 11 may beset again or repetitively in the course of process or the specimentreatment when the spectrum intensity of a certain wavelength changessignificantly in the course of the process being carried out.

FIG. 5 is a block diagram showing schematically and generally astructure of the emission spectroscopic processing apparatus accordingto a further embodiment of the present invention. The spectroscopicprocessing apparatus according to the instant embodiment of theinvention differs from the preceding embodiments in that the signals ofdifferent wavelengths are not added together but undergo the A/Dconversion after the amplification only. Needless to say, by setting thegain of the amplifier circuit 7 for each of the different wavelengths bymeans of the gain setting circuit 18, the S/N ratio for the componentsof low brightness can be improved, the effect of which is however lesssignificant when compared with the emission spectroscopic processingapparatus described previously by reference to FIG. 4 because of low S/Nratio of the analogue signal itself.

In the foregoing, description has been made of typical or preferredembodiments of the emission spectroscopic processing apparatus accordingto the present invention. By using the processing apparatus, it ispossible to detect at earlier stage of the process the minute andhigh-rate change of the spectral emission in the course of the plasmaprocessing. By way of example, in the case of the plasma processingapparatus employed for gate etching process of a semiconductor device inwhich the gate-length is not greater than 0.1 μm, the thickness of thebase or substrate insulation film subjected to the process is extremelythin as on the order of several nm to 1 nm. Such being thecircumstances, the plasma processing step has to be terminated in thestate where the above-mentioned film remains in a thickness of severalnm to several ten nm before all the etching-subjected film hascompletely been etched, whereon a succeeding plasma processing step hasto be started on the other conditions which can ensure high selectivityratio relative to the substrate.

For measuring the remaining amount of the etching-subjected filmmentioned above, it is necessary to observe or examine interferencelight from the wafer. However, in the case of this method, the change oflight on a wavelength basis is as small as on the order of 0.1 % toseveral %. By contrast, in the case where the spectroscopic processingapparatus described hereinbefore is employed, the S/N ratio of thesignal can significantly be improved, and the plasma processing step canbe stopped or terminated in correspondence to a high response of onesecond or less. Thus, the etching process can be performed for the gatelength shorter than 0.1 μm inclusive.

On the other hand, in the case where several thousands of wafers are tobe treated in succession by the etching process, it is required to knowthe changes within the process by observing observe the change of lightquantity in the state where the resolution of the wavelength isincreased. However, high-speed response performance is not necessarilyrequired. In this sort of application, the output signal of theamplifier circuit 7 shown in FIG. 3 is selected by means of the analogueswitching circuit 13 to be inputted to the signal processing unit 9 byway of the A/D converter 8. In other words, for this kind ofapplication, resolution of the wavelength is required. Accordingly, theinter-wavelength averaging is not performed but a plurality of sampleddata are averaged. Through this procedure, the S/N ratio can be improvedin the course of the signal processing.

It is assumed, by way of example, that operation for sampling one datafor each wavelength on a 0.5-second basis is carried out for one minute.Then, 120 pieces of data can be sampled for each of the wavelengths. Byaveraging these sampled data on a wavelength basis, the S/N ratio can beimproved by a factor of √{square root over (120)}=10.9 (i.e., by√{square root over (120)}=10.9 times). As can now be appreciated, boththe detection of minute change which does not require the wavelengthresolution but requires the response rate not longer than one second onone hand and the detection of minute change which requires thewavelength resolution with the response rate on the order of several tenseconds on the other hand can be carried out with one and the sameapparatus of the structure described hereinbefore by reference to FIG.3.

Further, the present invention can find application equally to thedetection of minute change of the emission component spectra whichrequires a high-speed response of shorter than 1 second inclusive aswell as the monitoring of minute changes of the individual wavelengthcomponents of the emission spectra. Furthermore, abnormality of theplasma processing can be prevented in advance by issuing an abnormalitysignal, an alarm display or terminating a succeeding treatment in thecase where the rate of the change should exceed a predetermined value.

As can now be understood from the foregoing, according to the teachingsof the present invention incarnated in the illustrated embodiments, itis possible to process speedily and stably the minute changes (notgreater than 10%) of the component wavelengths emitted during the plasmaprocessing in the timing within one second inclusive (preferably shorterthan 0.5 second inclusive). Besides, the mode in which the minutespectral change in each of the wavelengths emitted in the course ofplasma processing is processed stably at a high speed and the mode inwhich the spectral changes for each of the wavelengths emitted duringthe plasma processing are determined with high resolution for theadjacent wavelengths, respectively, can be carried out with a singleapparatus by changing over the modes mentioned above in dependence onthe applications as desired.

Next, description will be made of the digital processing of the lightsignal obtained from the CCD array. At first, description will bedirected to the features or characteristics of the plasma emission inthe plasma etching process. In the plasma etching process carried outwithin a vacuum process chamber, Cl₂, HBr, CF₄, C₅F₈ and the like gasesare used as the processing gas (reactive gas), while an Ar-gas isemployed for intensifying ionization of the plasma. These gases aredecomposed into Cl—, Br—, F-atoms (radicals) exhibiting high reactivityby the plasma. These radical gases react with silicon (Si), polysilicon(Si), oxide film (SiO₂), nitride film (Si₃N₄), BARC (BackAnti-Reflection Coating), Pt, Fe, SBT (SrBi₂Ta₂O₉) and the like whichare materials to be etched, to thereby produce reaction products such asSiCl, SiCl₂, SiF, SiBr, C₂, CO, CN, PtCl, FeCl, TaCl and the like as theetching process proceeds. When the material(s) to be etched becomesunavailable, i.e., when the etching process comes to an end, thereaction products are no more produced and decrease while the radicalgases increase.

Thus, intensities of the emission spectra in the course of plasmaetching process can be classified into (1) spectra due to the reactionproducts which decrease at the time point when the etching of thematerials to be etched is terminated, (2) spectra due to the radicalswhich increase at the end point of the etching, and (3) spectra due tomaterials irrelevant to the etching reaction and undergoing no changebefore and after the end of the etching (i.e., around the end point ofetching).

In the method of determining the end point based on the plasma emission,time-dependent change of the emission intensity of a specific spectrumwavelength (e.g. spectrum due to the reaction product) is used among theemission spectra mentioned above. However, since the emission spectrumsignals derived from the output of the CCD array contain noisecomponents in dependence on the signal intensity, as describedhereinbefore, the noise components make it difficult to detect the endpoint of the etching process with the method in which the differentialwaveform of the emission spectrum signal is made use of for detectingthe end point of the etching process.

In the following, referring to FIGS. 6 and 7, exemplary embodiments ofthe present invention designed for eliminating the noise components willbe described. It is assumed that the digitized signal resulting fromdigitization of the emission spectrum signal of wavelength λ derivedfrom the output of the CCD array at a time point t by means of the A/Dconverter 8 is expressed in terms of a light signal component i(λ, t)and a noise component δi(λ, t). In this conjunction, the noisecomponents δi(λ, t) are attributable to electrical noise in the CCDarray and fluctuation noise of light. The emission spectrum signal i(λ,t)+δi(λ, t) is temporarily saved in a digitized data hold circuit 910incorporated in the signal processing unit 9 mentioned hereinbefore,whereon the above-mentioned emission spectrum signal i(λ, t)+δi(λ, t)and a mask function M(λ) are added together for all the wavelengths λmeasured by using preset values of an emission spectrum classifying maskfunction M(λ) circuitry 911 by means of an arithmetic circuitry 912. Theintegrated or sum value ΣM(π)[i(λ, t)+δi(λ, t)] for λ is stored in a sumvalue hold circuit 913, whereon time-dependent differential value of theemission intensity is determined by means of a differentiationprocessing circuit 914. By making use of this time-dependentdifferential value of the emission intensity, the end point of theetching process is determined by a differential value decision circuit915. A processing flow to this end is illustrated in FIG. 7. Referringto the figure, the mask function M(λ) is set for the CCD wavelengths ina step 801 by inputting the etching conditions such as of materials tobe etched and the processing gases (step 800). Subsequently, uponstarting of the etching process, sampling of the light signal outputtedfrom the CCD array is started (step 802), whereby light signals i(λ,t)+δi(λ, t) for the individual wavelengths λ derived from the output ofthe CCD are acquired (step 803). In succession, the integrated or sumvalue ΣM(λ)[i(λ, t)+δi(λ, t)] of the light signal and the mask functionM(λ) for all the wavelengths λ is arithmetically determined (step 804).On the basis of this integrated or sum value ΣM(λ)[i(λ, t)+δi(λ, t)],the time-dependent differential value of the emission intensity at thetime point t is determined (step 805). By comparing this time-dependentdifferential value with a preset differential value as reference fordecision (step 806), the light signal i(λ, t)+δi(λ, t) is again acquired(step 803) or alternatively the etching process or the light signalsampling is terminated (step 807). In conjunction with the integrated orsum value of λ, i.e., ΣM(λ)[i(λ, t)+δi(λ, t)], it is to be noted thatthe term Σ[δi(λ, t)] represents random noise. Consequently, the sumvalue resulting from the summation of a large number of wavelengths λapproaches to zero. In other words, through this summation process,there arises the possibility of noise elimination.

Next, description will be made as to the method for classification ofthe emission spectra. For determining discriminatively whether theemission spectrum of wavelength λ derived from the output of the CCDarray is the light signal which decreases around the end point of theplasma etching process or alternatively the light signal which increasesor alternatively the light signal which undergoes no change, there canbe adopted the methods described below.

(1) A database of the reaction products of the reactive gases and thematerials to be etched is prepared in advance on the basis of a spectrumlibrary (see literature: CRC Handbook of Chemistry and Physics, David R.Lide, CRC Press, R. W. Pearse and A. G. Gaydon, “THE IDENTIFICATION OFMOLECULAR SPECTRA”, John Wiley & Sons, Inc., 1976), and by referencingthe database, the wavelengths belonging to the reactive gases areclassified as the wavelengths increasing before and after (i.e., around)the end point of the etching process while the wavelengths belonging tothe reaction products are wavelengths, around, the end point of theetching process with the other wavelengths being classified as thoseirrelevant to the reaction and undergoing essentially no time-dependentchange.

(2) A sample wafer processing (etching process of wafers containing samespecies of the materials to be etched is performed. In conjunction withthe time-dependent changes of the emission spectra in the etchingprocess, differentiation processing is performed for all thewavelengths, whereon the wavelengths are classified on the basis of thefirst-order differential values around the etching end point. As thedifferentiation processing method to this end, a method described inJapanese Patent Application Laid-Open Publication No. 228397/2000(JP-A-12-228397) may be adopted. More specifically, the wavelength forwhich the first-order differential value is negative (minus) isclassified as the wavelength which decreases around the etching endpoint (i.e., wavelength attributable to the reaction product), thewavelength for which the first-order differential value is positive(plus) is classified as the wavelength which increases around theetching end point (i.e., wavelength attributable to the radical), whilethe wavelengths whose first-order differential value is zero isclassified as the wavelength which undergoes no change around the endpoint of the etching process (i.e., the wavelength irrelevant to thereaction).

(3) A sample wafer processing (etching process of wafers containing samespecies of the materials to be etched) is performed. In conjunction withthe time-dependent changes of the emission spectra for all thewavelengths in the etching process, the main component analysis isperformed to determine spectra of the individual components, whereon thewavelengths are classified on the basis of the spectrum values of theindividual components. Concerning the analysis of the principalcomponents, reference is to be made to S. Minami: “WAVEFORM DATAPROCESSING FOR SCIENTIFIC MEASUREMENTS” CQ publication company of Japan,pp. 220-226 (1986) and K. Sasaki, S. Kawata and S. Minami, “ESTIMATIONOF COMPONENT SPECTRAL CURVES FROM UNKNOWN MIXTURE SPECTRA”, Appl. Opt.Vol. 23, pp. 1955-1959 (1984). The method disclosed in these referencescan be adopted in carrying out the present invention. The wavelengthsare classified on the basis of the spectrum values of given componentsdetermined by the principal component analysis. By way of example, thewavelength for which a spectrum value of a certain component is negativeis classified as the wavelength which decreases around the etching endpoint (i.e., wavelength attributable to the reaction product), thewavelength for which the spectrum value is positive is classified as thewavelength which increases around the etching end point (i.e.,wavelength attributable to the radical), while the wavelength whosespectrum value is zero is classified as the wavelength which undergoesno change around the end point of the etching (i.e., the wavelengthirrelevant to the reaction). It should however be noted in conjunctionwith the methods described above that the positive spectrum value is notalways attributable to the reaction product and that negative spectrumvalue is not always that of the radical either.

In order to discriminatively identify three groups resulting from theclassification through the procedures described above, operator M(λ) isintroduced. By way of example, for the wavelength λ whose intensityincreases around the etching end point is assigned with the operatorM(λ)=−1, while the wavelength λ whose intensity decreases around theetching end point is assigned with the operator M(λ)=1. Further, thewavelength X whose intensity undergoes no change around the etching endpoint is assigned with the operator M(λ)=1.

FIGS. 8 and 9 show results of BARC (Back Anti-Reflection Coating)etching process to which the teaching of the invention incarnated in theinstant embodiment is applied. The etching process gas is a gas mixtureof HBr, CF₄, O₂ and Ar. In FIG. 8, there are graphically illustrated, byway of example only, the emission spectra before and after the BARCetching, respectively. As can be seen in FIG. 8, many emission spectraof CN, CO, C₂ and others which are reaction products in the BARC processdecrease around the time point at which the etching is ended, whileemission spectra of OH and O increase. By performing differentiationprocessing on the time-dependent change (i.e., change as a function oftime lapse), the time-dependent change behaviors of the emission spectraare classified to thereby determine the mask function M(λ) shown in thefigure. The sum ΣM(λ)[i(λ, t)+δi(λ, t)] determined concerning thewavelength λ by using the mask function M(λ) is illustrated in FIG. 9.Referring to this figure, in the standard state, the mean value of theemission spectrum intensities is about 1220 counts, while in the statewhere the emission quantity decreases by 1/100, the squeezed mean valueof the emission spectrum intensities is about 12.2 counts. As can beseen, according to the teachings of the present invention, thetime-dependent change of the emission intensity can be determined withsufficiently high accuracy even in the state where the emission quantityhas decreased by 1/100 because noise components can sufficiently beeliminated.

Next, description will be made of an embodiment of the invention inwhich the trend of the noise components δi(λ, t) outputted from the CCDarray being in reverse proportion to the intensity of the light signalcomponent i(λ) is taken into consideration. In the experiments asconducted, a CCD array commercially available from Sony Ltd. under thecommercial name “ILX 511” was used. The S/N ratio of this CCD can beexpressed as ca. 250 √{square root over ((i(λ)/4000))}, while the noisecomponent can be given by δi(λ, t)= 1/250* 1√{square root over((4000*i(λ, t)))}. For example, when i(λ)=4000, noise can be given byδi(λ, t)=16(S/N=250). On the other hand, for i(λ)=10, δi(λ, t)=0.8(S/N=12.5). When integration or summation operation is performed simplyfor λ, contribution to the S/N ratio of the emission quantity isneglected. This can be taken into account by standardizing theintegrating or summing operation ΣM(λ)[i(λ, t)+δi(λ, t)] performed on λwith the noise component δi(λ, t). In other words, for λ shown in FIG.8, the sum 804 may be determined as ΣM(λ)[i(λ, t)+δi(λ, t)]/[1/250*√{square root over ((4000*i(λ,t)))}] (see a step 814 in the flowchart shown in FIG. 10).

Finally, description will be directed to an embodiment of the inventionwhich is so arranged as to cancel out fluctuations or changes such asthose ascribable to abnormal discharge of the plasma emission, if itoccurs. The value of the mask function M(λ) mentioned previously is, forexample, set as follows. For the wavelength λ for which the emissionintensity increases around the end point of the etching process is setequal to “2”, i.e., M(λ)=2. On the other hand, for the wavelength λ ofwhich emission intensity decreases around the end point of the etchingprocess is set to “−2”, i.e., M(λ′)=−2. Further, for the wavelength λwhose emission intensity undergoes substantially no change is set to“1”, i.e., M(λ)=“1”. In this way, the wavelength (λ) for which emissionintensity undergoes no change around the end point of the etchingprocess can be distinguished from the other wavelengths, and the lightsignal of this wavelength λ can be used for standardization of thesummation of λ, i.e., ΣM(λ)[i(λ, t)+δi(λ, t)]. In other words, summingA*ΣM(λ)[i(λ, t)+δi(λ, t)]/ΣM(λ′)[i(λ′, t)+δi(λ′, t)] is arithmeticallydetermined. In this conjunction, the term ΣM(λ′)[i(λ′, t)+δi(λ′, t)]represent the sum value for the wavelength λ′ whose emission intensitydoes not change around the end point of the etching. Further, thecoefficient integration represents the value of ΣM(λ′)[i(λ′, t)+δi(λ′,t)]/ ΣM(λ)[i(λ, t)+δi(λ, t)] at an appropriate time point to after theplasma etching process has been started. Through this standardization ornormalization processing (see step 824 in the flow chart of FIG. 11), itis possible to cancel out fluctuations of emission spectra possiblybrought about by abnormal electric discharge, and thus determination asto the end point of the etching process can be made with high accuracyand reliability.

Further, in the case where the wavelength for which emission intensitydoes not change around the end point of the etching process is absent,then the integration or summation value for the wavelength whoseemission intensity increases around the end point of etching can be usedfor the standardization or normalization substantially to the similaradvantageous effect. In other wards, addition of λ′ in the summationA*ΣM(λ)[i(λ, t)+δi(λ, t)]/ΣM(λ′)[i(λ′, t)+δi(λ′, t)] may be performedfor the wavelength λ of which emission intensity increases around theend point of the etching process.

As is apparent from the above, the present invention has provided theemission spectroscopic processing apparatus which is capable ofdetecting high-rate minute changes of emission spectra with improvedreproducibility.

It should be further understood by those skilled in the art that theforegoing description has been made on embodiments of the invention andthat various changes and modifications may be made in the inventionwithout departing from the spirit of the invention and the scope of theappended claims.

1. A plasma processing method using a spectroscopic processing unit,comprising the steps of: a) separating spectrally plasma radiationemitted from a vacuum process chamber into component spectra; b)converting said component spectra into a time series of analogueelectric signals composed of different wavelength components at apredetermined period; c) adding together analogue signals of thedifferent wavelength components; d) converting a plurality of pluraladded said analogue electric signals into digital signals on apredetermined period basis; e) adding, for each of at least two ofpredetermined plural kinds of materials within said vacuum processchamber, said digital signals of a set of wavelengths corresponding to aset of emission spectrum wavelengths intrinsic to the material; f)determining discriminatively an end point of a predetermined plasmaprocess on a basis of said added signals obtained in said step e); andg) terminating said predetermined plasma process.
 2. A plasma processingmethod according to claim 1, wherein in step c), a frequency range ofthe different wavelength components of the analogue electric signals tobe added is variable.
 3. A plasma processing method according to claim1, wherein in step a), plasma radiation emitted from each of a pluralityof vacuum process chambers is separated spectrally into componentspectra, and steps b) to g) are performed for plasma radiation beingseparated spectrally from each of said plurality of vacuum processchambers.
 4. A plasma processing method using a spectroscopic processingunit, comprising the steps of: a) separating spectrally plasma radiationemitted from a vacuum process chamber into component spectra; b)converting said component spectra into a time series of analogueelectric signals composed of different wavelength components at apredetermined period; c) adding together analogue signals of thedifferent wavelength components; d) converting a plurality of pluraladded said analogue electric signals into digital signals on apredetermined-period basis; e) adding, for each of at least two ofpredetermined plural kinds of materials within said vacuum processchamber, said digital signals of a set of wavelengths corresponding to aset of emission spectrum wavelengths intrinsic to the material; f)performing an adding or subtracting operation between said added signalsobtained in step e) as to said at least two of said predetermined pluralkinds of materials; g) determining discriminatively an end point of apredetermined plasma process on a basis of a signal resulting from saidstep f); and h) terminating said predetermined plasma process.
 5. Aplasma processing method according to claim 4, wherein in step c), afrequency range of the different wavelength components of the analogueelectric signals to be added is variable.
 6. A plasma processing methodaccording to claim 4, wherein in step a), plasma radiation emitted fromeach of a plurality of vacuum process chambers is separated spectrallyinto component spectra, and steps b) to h) are performed for plasmaradiation being separated spectrally from each of said plurality ofvacuum process chambers.